1. Field of the Invention
The present invention relates generally to an embedded passive device structure and manufacturing method thereof, and in particular, to an embedded passive device structure of higher electric value (such as, for example, capacitance) and manufacturing method thereof.
2. The Prior Arts
The so-called embedded passives or embedded passive devices, through using the thin core manufacture of the multilayer PCB, the etching or the printing processes are thereby carried out. And then the capacitors or the resistors are manufactured on the thin core directly. When after being laminated to become the multilayer PCB, the discrete passive devices which are welded on top of the board during assembly fabrication are thereby replaced, so as to save on board space for use by the active devices and their corresponding circuits.
The embedded, implanted or the buried technology which is started by the Ohmega-ply Company uses the matte side of the original copper-cladding on the innerlayer surface, and performs processing of the nickel phosphorus alloy layer at the upper thin film layer, which is regarded as the resistive element, and lamination to form the thin core. Then, the photoresist technology is used twice, as well as performing etching three times to form the required film “resistor” on a specific designated position. Because the above resistor is embedded inside the thin core, it is commonly known also as a Buried Resistor (BR).
Thereafter, in 1992, a PCB manufacturer in the U.S.A named Zycon® adds in an extremely thin (2-4 mil) dielectric layer as the thin core at the upper layers inside the multilayer PCB, which is outside of the original Vcc/GND thin core, and uses the vast area of the parallel copper sheet/foil of the thin core for manufacturing an integrated capacitor, with the tradename of Buried Capacitor™ (BC). It has advantages such as interference reduction, offering more charge capacity, and providing steady voltage while operating under fundamental frequencies. Zycon® Company had applied for several patents relating to the BC (namely, the U.S. Pat. Nos. 5,079,069, 5,161,086, and 5,155,655).
Generally, if needed to improve the capacitance of the capacitor in a case of under a stable voltage condition, other than the adoption of a high-k dielectric layer, a larger area of the source electrode, and of the ground electrode are also adopted. In other words, the source electrodes and ground electrodes should be as close to each other as possible. However, if the embedded capacitor structure from the Zycon® Company, were adopted because of the desire to adopt the ultra thin substrate, the manufacturing process is thereby difficult, and the yield is lower at the same time. Inside this capacitor structure, the source electrode, the ground electrode, and the dielectric layer are sequentially stacked. In addition, for achieving higher capacitance, the ground electrode and the source electrode are already very much close to each other. Thereafter, during the lamination process, because of the ease of exerting force upon the upper layers of the source or ground electrodes, it is conceivable to be able to embed some of the source electrodes or the ground electrodes into the dielectric layer, so as to make the source electrode and the ground electrode to form contact with each other, thereby resulting in fabricating the bad products. Although the conventional manufacturing technology was already dramatically improved upon by using the Zycon® method, however, under the general design criteria, the thickness of the dielectric layer at the minimum remains to be at 12 μm. As for adopting the high-k resin, the difficult part lies within the difficulty to control the final thickness during lamination after the printing steps, and to require to be using the copper sheet having low profile to avoid the problem of producing cohesion at the same time, such method is to make the reliability of the circuit board being put to the test.